Preschool symptoms of attention-deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) mark significant risk for the development of early-onset conduct disorder (CD). In turn, early-onset CD exacts enormous costs on society in terms of delinquency, school drop out, substance abuse, and interpersonal violence. Because CD interventions become increasingly less effective with older children, preventing the disorder may require that we target at-risk preschoolers with ADHD, before conduct problems emerge. Yet very little research has addressed ADHD among preschoolers, with no studies examining CD outcomes. Although psychostimulants are effective in reducing core ADHD symptoms, they do not appear to prevent the progression of ADHD and ODD to CD. Given this, and given that psychosocial interventions represent the only well established CD treatment, it is critical to evaluate such interventions for preschoolers with ADHD, and to examine CD outcomes. It is also critical that we examine the biological substrates of impulsivity and emotion regulation in preschoolers with ADHD, as it remains unclear whether markers that differentiate between older children with ADHD and CD also discriminate between ADHD preschoolers who do and do not develop later CD, or if these markers diverge later in development. Furthermore, it is unknown whether such markers predict treatment response or are affected by successful intervention. In the proposed study, a multimodal intervention of established efficacy for treating conduct problems will be delivered to 4- to 6-year-old children with ADHD or ADHD + ODD. Aims of the study are to (a) evaluate the effectiveness of the intervention for interrupting the ongoing progression of ADHD/ODD to CD;(b) assess pretreatment biological markers of impulsivity and emotion regulation and determine if such markers predict treatment response;(c) examine biological and psychosocial mediators of treatment outcome at 2-year follow-up;(d) examine familial mediators of children's behavioral and physiological regulation at follow-up. The study will be the first to (a) test whether a well established intervention for ODD and CD is effective in preventing CD in high-risk preschoolers, (b) assess pretreatment biological markers of outcome, (c) examine interactions between biological and environmental risk for developing CD, and (d) examine potential differences in CD outcomes for ADHD preschoolers with and without comorbid oppositionality.